To produce polyethylene with preset properties meeting the requirements of processing and operation of the articles, a great importance is attached to molecular weight of polyethylene and, in particular, to molecular-mass distribution (MMD).
It is a widespread opinion that polyolefins obtained on Ziegler-Natta catalysts have a high MMD (more than 10) (H. Wesslau, Makrom, Chem., 20, 111, 1956), but as early as 1958, the same author (H. Wesslau, Makrom. Chem., 26, 102, 1958) published the data which show that polyethylene with MMD about 2 can be obtained on said catalytic systems.
In many patents it is proposed to introduce different additives for narrowing or widening MMD. Such additives can be alcohols (U.S. Pat. No. 3,163,611), phenols (U.S. Pat. No. 3,150,122), organic and inorganic acids and the salts thereof (U.S. Pat. Nos. 2,912,425 and 3,682,198; British Pat. No. 975,675), water (U.S. Pat. Nos. 3,184,416 and 3,440,237; FRG Pat. No. 1,022,382; French Pat. No. 1,361,252), organic oxides and esters (FRG Pat. No. 1,098,715).
Known in the art is a process for producing polyethylene by polymerization of ethylene in a medium of a hydrocarbon solvent at a temperature within the range of from -20.degree. to +150.degree. C. under a pressure of from 1 to 70 atm.g. in the presence of a complex organometallic catalyst consisting of alkylaluminiumhalide of the general formula AlR.sub.n (X).sub.3-n (I), wherein R is an alkyl with a number of carbon atoms from 1 to 10, X is Cl, Br, Y, or F, n=1 or 2; oxyvanadiumalkoxide of the formula OV(OR').sub.3 (II), wherein R' is an alkyl with a number of carbon atoms from 1 to 20, at a molar ratio between the catalytic components I:II equal to 5-100:1, respectively (U.S. Pat. No. 3,313,794).
Polyethylene produced by the above known method has a number of advantages as compared with polyethylene obtained on other catalytic systems (for example, on a complex organometallic catalyst consisting of diethylaluminiumchloride Al(C.sub.2 H.sub.5).sub.2 Cl and titanium tetrachloride TiCl.sub.4): high impact viscosity and tensile strength, higher transparency and brilliance of the film surface, etc. This makes it possible to widely use this polyethylene in making linear articles, reinforcement for water supply for engineering purposes; in machine-engineering; noiseless gears in high-speed machines with a low load; in automobile construction; fuel tanks, etc.
Said known method of producing polyethylene has a disadvantage in that the final polyethylene has a narrow MMD equal to 2-3. Organometallic catalysts of the same type as those described in the known method give polymers with a narrow MMD (W. L. Carrick et al., J. Amer. Chem. Soc., 81, 3883, 1960; E. A. Fuschman et al., Izv. AN SSSR, ser. khim., 1965, 2075). Polyethylene with a narrow MMD is used mainly for manufacturing articles by moulding (M. Leegwater, SPE Journal, 25, 47, 1969). For manufacturing articles by extrusion (for instance, cable or pipes) polyethylene is required with medium (5-7) and high (above 10) MMD (Karasev et al., Plastic materials, 1974, 6, 40), especially in the case of highly molecular polyethylenes (T. Boiangin, Mat. Plastics, III, No. 3, 153, 1965; No. 2, 88, 1966). In addition, this method is disadvantageous in that the produced polyethylene has a low adhesion to metal (for instance, the adhesion value of the polymer to an oxidized copper foil is 800 g/cm) which is one of the important characteristics in the production of adhesives.
It is an object of the present invention to eliminate the above-cited disadvantages.